The invention relates generally to integrated circuits and, in particular, to device structures for a field-effect transistor and methods of forming a device structure for a field-effect transistor.
Complementary-metal-oxide-semiconductor processes may be used to build a combination of p-type and n-type field-effect transistors that are used to construct logic gates and as active components in other types of circuits, such as switches used in radiofrequency circuits. Device structures for a field-effect transistor generally include a source, a drain, and a gate electrode configured to switch carrier flow in a channel region arranged between the source and drain. When a control voltage exceeding a designated threshold voltage is applied to the gate electrode, carrier flow occurs in the channel region to produce a device output current.
A silicon-on-insulator substrate permits device operation at significantly higher speeds with improved electrical isolation and reduced electrical losses in comparison with field-effect transistors built using a bulk silicon wafer. Generally, a silicon-on-insulator substrate includes a thin device layer of semiconductor material, a substrate, and a buried oxide layer physically separating and electrically isolating the device layer from the substrate. Contingent on the thickness of the device layer, a field-effect transistor may operate in a fully-depleted mode in which a depletion layer in the channel region extends fully to the buried oxide layer when typical control voltages are applied to the gate electrode.
The channel region of a fully-depleted p-type field-effect transistor may be composed of silicon-germanium. A conventional approach for forming the silicon-germanium channel region is to grow an epitaxial silicon-germanium layer on a section of the device layer and to perform a thermal condensation process. Germanium is transported from the epitaxial silicon-germanium layer to the device layer and throughout the channel region down to the buried oxide layer. The silicon-germanium channel region formed by the thermal condensation process may be prone to strain relaxation and broken bonds at an interface with the buried oxide layer. The broken bonds may elevate the interface trap density at the interface, and the strain relaxation may impact device performance through a loss of carrier mobility.
Improved device structures for a field-effect transistor and methods of forming a device structure for a field-effect transistor are needed.